Metrology Methods, Radiation Source, Metrology Apparatus and Device Manufacturing Method

ABSTRACT

A target structure (T) made by lithography or used in lithography is inspected by irradiating the structure at least a first time with EUV radiation ( 304 ) generated by inverse Compton scattering. Radiation ( 308 ) scattered by the target structure in reflection or transmission is detected ( 312 ) and properties of the target structure are calculated by a processor ( 340 ) based on the detected scattered radiation. The radiation may have a first wavelength in the EUV range of 0.1 nm to 125 nm. Using the same source and controlling an electron energy, the structure may be irradiated multiple times with different wavelengths within the EUV range, and/or with shorter (x-ray) wavelengths and/or with longer (UV, visible) wavelengths. By rapid switching of electron energy in the inverse Compton scattering source, irradiation at different wavelengths can be performed several times per second.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/747,499, filed Jan. 25, 2018, which is a National Stage Entry ofinternational Application No. PCT/EP2016/068479, filed Aug. 3, 2016,which claims priority of European Application No. 15180740.1, filed onAug. 12, 2015 and is incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for metrologyusable, for example, in the manufacture of devices by lithographictechniques and to methods of manufacturing devices using lithographictechniques. Methods of measuring critical dimension (line width) aredescribed, as a particular application of such metrology. For thepurpose the present disclosure, metrology includes inspection for a widerange of purposes, including for example detection of defects, and notonly to quantitative measurement of a particular dimension or materialproperty.

BACKGROUND ART

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwater),

In lithographic processes, it is desirable frequently to makemeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes (SEM), which are often used tomeasure critical dimension (CD). Other specialized tools are used tomeasure parameters related to asymmetry. One of these parameters isoverlay, the accuracy of alignment of two layers in a device. Recently,various forms of scatterometer have been developed for use in thelithographic field. These devices direct a beam of radiation onto atarget and measure one or more properties of the scatteredradiation—e.g., intensity at a single angle of reflection as a functionof wavelength; intensity at one or more wavelengths as a function ofreflected angle; or polarization as a function of reflected angle toobtain a “spectrum” from which a property of interest of the target canbe determined. Determination of the property of interest may beperformed by various techniques: e.g., reconstruction of the targetstructure by iterative approaches such as rigorous coupled wave analysisor finite element methods; library searches; and principal componentanalysis. Compared with SEM techniques, optical scatterometers can beused with much higher throughput, on a large proportion or even all ofthe product units.

As technology develops, however, performance specifications become evertighter. A further limitation of current methods is that they are madewith optical wavelengths, requiring dedicated metrology structures withdimensions much greater than the typical dimensions of real productfeatures. Accordingly, the measurements made on these metrologystructures are only indirectly indicative of the real productstructures. A particular parameter of interest is linewidth (CD), and asuitable small-target method for CD measurement has not yet beendevised.

To obtain higher resolution measurements, it has also been considered touse EUV radiation, with wavelengths in the range for example 0.1 to 125nm. EUV radiation is particularly attractive as it has wavelengths ofthe same order as the structures to be measured. Spectroscopic EUVreflectometry, for example, is proposed in the European patentapplication number 15160786, not published at the present priority date.Unfortunately, due to limitations of radiation sources available, thereis no existing technique that provides for metrology on small targets,such as an in-die grating, or product structures themselves with a speedsuitable for mass measurement in high-volume manufacturing. An idealradiation source would be compact and affordable, and have a highbrightness coupled with free selection of wavelength and good ability tofocus into a small target area.

A bright, compact x-ray source has recently been described based on thephenomenon of inverse Compton scattering (ICS). This is described by W SGraves et al, in “Compact x-ray source based on burst-mode inverseCompton scattering at 100 kHz”, Physical Review SpecialTopics—Accelerators and Beams 17, 120701 (2014). The contents of theGraves et al reference and associated patent application areincorporated herein by reference. To reach high brightness on theelectrons, a linear accelerator is used to the desirable high brightnessfor use in x-ray metrology applications. Details of a linear acceleratorused in the x-ray source are provided in published patent applicationUS2014191654A1 (Tantawi & Neilson). Other ways to accelerate theelectrons are in development by other workers.

SUMMARY OF THE INVENTION

The invention aims to provide alternative methods of small targetmetrology, overcoming one or more of the drawbacks of the optical andX-ray methods described above. A particular desire is to measureparameters for example at locations within product areas on asemiconductor substrate, while improving the speed with which suchmeasurements can be performed and while adapting to the smallerdimensions of features made by current and future lithographictechnologies.

The inventors have recognized that a source based on inverse Comptonscattering can be adapted to provide a bright and controllable source inthe EUV (soft X-ray) range, to enable high-volume metrology on smalltargets. The same adapted source can be used to inspect other types ofstructures, both natural and man-made, not only semiconductor products.

The invention in a first aspect provides a method of measuring aproperty of a structure, the method comprising irradiating the structureat least a first time with radiation, detecting said radiation afterinteraction with the structure and determining a property of thestructure based on a property of the radiation, wherein said radiationis generated by inverse Compton scattering, the radiation having a firstwavelength in the range of 0.1 to 125 nm.

The invention further provides a radiation source apparatus based oninverse Compton scattering, the apparatus comprising an electron sourceand a photon source, and a controller for controlling the electronsource and the photon source to deliver one or more bunches of electronsto an interaction point at the same time as a pulse of photons, wherebya proportion of the photons acquire additional energy by inverse Comptonscattering to and are output by the apparatus, wherein the additionalenergy is controllable such that the photons output by the apparatushave a wavelength in the range 0.1 nm to 125 nm.

The inventors have recognized that a source based on inverse Comptonscattering can be designed and controlled to deliver radiation of anextremely wide range of wavelengths, not only EUV wavelengths and/orx-rays. The same source, for example, may be operable to provideradiation in EUV, UV, and even visible wavelengths, and/or x-raywavelengths, in addition to radiation at EUV wavelengths.

The invention further provides a metrology apparatus for use inmeasuring properties of a structure, the apparatus comprising:

a radiation source apparatus according to the invention as set forthabove;

an illumination system for delivering photons output by the radiationsource apparatus in a beam of radiation onto the structure; and

a detection system for detecting radiation from the structure after saidphotons have interacted with the structure.

In a particular implementation, the apparatus is adapted to receivesemiconductor waters (for example 300 mm wafers) from an automated waferhandler. In other applications, the apparatus can be adapted formeasurement of any type of structure, whether natural or man-made.

In yet another aspect, the invention provides a device manufacturingmethod comprising:

transferring a pattern from a patterning device onto a substrate using alithographic process, the pattern defining at least one periodicstructure;

measuring one or more properties of the periodic structure to determinea value for one or more parameters of the lithographic process; and

applying a correction in subsequent operations of the lithographicprocess in accordance with the measured property,

wherein the step of measuring the properties of the periodic structureincludes measuring a property by a method according to the invention,set forth above.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings in which:

FIG. 1 depicts a lithographic apparatus together with other apparatusesforming a production facility for semiconductor devices;

FIG. 2 illustrates the geometry of incident and reflected rays inrelation to a grating target in a metrology method according to a firstembodiment of the present invention;

FIG. 3 illustrates schematically the components of a metrologyapparatus, performing the method of FIG. 2;

FIG. 4 shows schematically the construction of a radiation source usedin the apparatus o FIG. 3;

FIG. 5 shows in more detail a part of the radiation source of FIG. 4;

FIG. 6 (a) illustrates by a schematic side view the elongation of a spotof radiation under grazing incidence, with schematic representations ofbeam cross-section B an spot S shown schematically at FIG. 6(b) and FIG.6(c) for different angles of incidence;

FIG. 7 illustrates schematically the components of an illuminationsystem in one embodiment of the apparatus of FIG. 3;

FIG. 8 illustrates variations of absorption for different materials overa range of wavelengths within part of the EUV spectrum;

FIGS. 9 to 12 illustrate various modes of application of the apparatusof FIG. 3;

FIG. 13 illustrates a modified apparatus using an ICS source to performtransmissive small angle x-ray scatterometry; and

FIG. 14 is a flow chart illustrating a method of controlling performanceof a metrology method and/or of a lithographic manufacturing processusing measurements made by the method of FIG. 14.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 at 200 shows a lithographic apparatus LA as part of an industrialfacility implementing a high-volume, lithographic manufacturing process.In the present example, the manufacturing process is adapted for themanufacture of for semiconductor products (integrated circuits) onsubstrates such as semiconductor wafers. The skilled person willappreciate that a wide variety of products can be manufactured byprocessing different types of substrates in variants of this process.The production of semiconductor products is used purely as an examplewhich has great commercial significance today.

Within the lithographic apparatus (or “litho tool” 200 for short), ameasurement station MEA is shown at 202 and an exposure station EXP isshown at 204. A control unit LACI) is shown at 206. In this example,each substrate visits the measurement station and the exposure stationto have a pattern applied. In an optical lithographic apparatus, forexample, a projection system is used to transfer a product pattern froma patterning device MA onto the substrate using conditioned radiationand a projection system. This is done by forming an image of the patternin a layer of radiation-sensitive resist material.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. The patterning MA device maybe a mask or reticle, which imparts a pattern to a radiation beamtransmitted or reflected by the patterning device. Well-known modes ofoperation include a stepping mode and a scanning mode. As is well known,the projection system may cooperate with support and positioning systemsfor the substrate and the patterning device in a variety of ways toapply a desired pattern to many target portions across a substrate.Programmable patterning devices may be used instead of reticles having afixed pattern. The radiation for example may include electromagneticradiation in the deep ultraviolet (DIN) or extreme ultraviolet (EUV)wavebands. The present disclosure is also applicable to other types oflithographic process, for example imprint lithography and direct writinglithography, for example by electron beam.

The lithographic apparatus control unit LACU which controls all themovements and measurements of various actuators and sensors to receivesubstrates W and reticles MA and to implement the patterning operations.LACU also includes signal processing and data processing capacity toimplement desired calculations relevant to the operation of theapparatus. In practice, control unit LACU will be realized as a systemof many sub-units, each handling the real-time data acquisition,processing and control of a subsystem or component within the apparatus.

Before the pattern is applied to a substrate at the exposure stationEXP, the substrate is processed in at the measurement station MEA sothat various preparatory steps may be carried out. The preparatory stepsmay include mapping the surface height of the substrate using a levelsensor and measuring the position of alignment marks on the substrateusing an alignment sensor. The alignment marks are arranged nominally ina regular grid pattern. However, due to inaccuracies in creating themarks and also due to deformations of the substrate that occurthroughout its processing, the marks deviate from the ideal grid.Consequently, in addition to measuring position and orientation of thesubstrate, the alignment sensor in practice must measure in detail thepositions of many marks across the substrate area, if the apparatus isto print product features at the correct locations with very highaccuracy. The apparatus may be of a so-called dual stage type which hastwo substrate tables, each with a positioning system controlled by thecontrol unit LACU. While one substrate on one substrate table is beingexposed at the exposure station EXP, another substrate can be loadedonto the other substrate table at the measurement station MEA so thatvarious preparatory steps may be carried out. The measurement ofalignment marks is therefore very time-consuming and the provision oftwo substrate tables enables a substantial increase in the throughput ofthe apparatus. If the position sensor IF is not capable of measuring theposition of the substrate table while it is at the measurement stationas well as at the exposure station, a second position sensor may beprovided to enable the positions of the substrate table to be tracked atboth stations. Lithographic apparatus LA may for example is of aso-called dual stage type which has two substrate tables WTa and WTb andtwo stations—an exposure station and a measurement station—between whichthe substrate tables can be exchanged.

Within the production facility, apparatus 200 forms part of a “lithocell” or “litho cluster” that contains also a coating apparatus 208 forapplying photosensitive resist and other coatings to substrates W forpatterning by the apparatus 200. At an output side of apparatus 200, abaking apparatus 210 and developing apparatus 212 are provided fordeveloping the exposed pattern into a physical resist pattern. Betweenall of these apparatuses, substrate handling systems take care ofsupporting the substrates and transferring them from one piece ofapparatus to the next. These apparatuses, which are often collectivelyreferred to as the track, are under the control of a track control unitwhich is itself controlled by a supervisory control system (SCS) 238,which also controls the lithographic apparatus via lithographicapparatus control unit LACU. Thus, the different apparatus can beoperated to maximize throughput and processing efficiency. Supervisorycontrol system SCS receives recipe information R which provides in greatdetail a definition of the steps to be performed to create eachpatterned substrate.

Once the pattern has been applied and developed in the litho cell,patterned substrates 220 are transferred to other processing apparatusessuch as are illustrated at 222, 224, 226, A wide range of processingsteps is implemented by various apparatuses in a typical manufacturingfacility. For the sake of example, apparatus 222 in this embodiment isan etching station, and apparatus 224 performs a post-etch annealingstep. Further physical and/or chemical processing steps are applied infurther apparatuses, 226, etc. Numerous types of operation can berequired to make a real device, such as deposition of material,modification of surface material characteristics (oxidation, doping, ionimplantation etc.), chemical-mechanical polishing (CMP), and so forth.The apparatus 226 may, in practice, represent a series of differentprocessing steps performed in one or more apparatuses.

As is well known, the manufacture of semiconductor devices involves manyrepetitions of such processing, to build up device structures withappropriate materials and patterns, layer-by-layer on the substrate.Accordingly, substrates 230 arriving at the litho cluster may be newlyprepared substrates, or they may be substrates that have been processedpreviously in this cluster or in another apparatus entirely. Similarly,depending on the required processing, substrates 232 on leavingapparatus 226 may be returned for a subsequent patterning operation inthe same litho cluster, they may be destined for patterning operationsin a different cluster, or they may be finished products to be sent fordicing and packaging.

Each layer of the product structure requires a different set of processsteps, and the apparatuses 226 used at each layer may be completelydifferent in type. Further, even where the processing steps to beapplied by the apparatus 226 are nominally the same, in a largefacility, there may be several supposedly identical machines working inparallel to perform the step 226 on different substrates. Smalldifferences in set-up or faults between these machines can mean thatthey influence different substrates in different ways. Even steps thatare relatively common to each layer, such as etching (apparatus 222) maybe implemented by several etching apparatuses that are nominallyidentical but working in parallel to maximize throughput. In practice,moreover, different layers require different etch processes, for examplechemical etches, plasma etches, according to the details of the materialto be etched, and special requirements such as, for example, anisotropicetching.

The previous and/or subsequent processes may be performed in otherlithography apparatuses, as just mentioned, and may even be performed indifferent types of lithography apparatus. For example, some layers inthe device manufacturing process which are very demanding in parameterssuch as resolution and overlay may be performed in a more advancedlithography tool than other layers that are less demanding. Thereforesome layers may be exposed in an immersion type lithography tool, whileothers are exposed in a ‘dry’ tool. Some layers may be exposed in a toolworking at DUV wavelengths, while others are exposed using EUVwavelength radiation.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. Accordingly a manufacturing facility in which litho cell LC islocated also includes metrology system MET which receives some or all ofthe substrates W that have been processed in the litho cell. Metrologyresults are provided directly or indirectly to the supervisory controlsystem SCS. If errors are detected, adjustments may be made to exposuresof subsequent substrates, especially if the metrology can be done soonand fast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped and reworkedto improve yield, or discarded, thereby avoiding performing furtherprocessing on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

Also shown in FIG. 1 is a metrology apparatus 240 which is provided formaking measurements of parameters of the products at desired stages inthe manufacturing process. A common example of a metrology apparatus ina modern lithographic production facility is a scatterometer, forexample an angle-resolved scatterometer or a spectroscopicscatterometer, and it may be applied to measure properties of thedeveloped substrates at 220 prior to etching in the apparatus 222. Usingmetrology apparatus 240, it may be determined, for example, thatimportant performance parameters such as overlay or critical dimension(CD) do not meet specified accuracy requirements in the developedresist. Prior to the etching step, the opportunity exists to strip thedeveloped resist and reprocess the substrates 220 through the lithocluster. As is also well known, the metrology results 242 from theapparatus 240 can be used to maintain accurate performance of thepatterning operations in the litho cluster, by supervisory controlsystem SCS and/or control unit LACLJ 206 making small adjustments overtime, thereby minimizing the risk of products being madeout-of-specification, and requiring re-work. Of course, metrologyapparatus 240 and/or other metrology apparatuses (not shown) can beapplied to measure properties of the processed substrates 232, 234, andincoming substrates 230.

Each generation of lithographic manufacturing technology (commonlyreferred to as a technology “node”) has tighter specifications forperformance parameters such as CD. One of the main challenges inmetrology is that the metrology target size is desired to be smallerthan the targets customarily used with metrology apparatus 240. Forexample, a present goal is to use targets with a size of 5 μm×5 μm orsmaller. These small sizes would permit wider use of so-called “in-die”metrology, where targets are located among the product structures(instead of being confined in scribe lane areas between product areas),or “on product” metrology, where the targets are the product structuresthemselves. The only metrology technique currently used for on-productCD metrology is electron microscopy (CD-SEM). This known technique showslimitations for future nodes, and only provides very limited geometricalinformation of the structure.

One approach to improving metrology of the smallest structures is to useshorter wavelengths of radiation, for example in the extreme ultraviolet(EUV), soft x-ray or even hard x-ray ranges. For example, EUVreflectometry, including spectroscopic EUV reflectometry, may beconsidered as a CD-metrology method for future technological nodes.X-ray scattering techniques such as small-angle x-ray scattering mayalso be considered, in transmissive mode (T-SAXS) or in grazingincidence mode (GI-SAXS). Principles and practice of EUV metrology inthis context are provided in the patent application EP15160786,mentioned above. There it is demonstrated that EUV reflectometry offersbenefits of high sensitivity, being robust against process variationsand being selective for a parameter of interest.

For the purpose of the present disclosure, hard x-rays are considered asrays with wavelength less than about 0.1 nm, for example including therange 0.01 to 0.1 nm. Soft-x-ray or EUV refers to the range extendingroughly from 0.1 nm to 125 nm wavelength. Different sub-ranges of theseranges can be chosen to suit the dimensions of structures underinvestigation. For example, for semiconductor structures at the limitsof current lithographic techniques, wavelengths in the range 0.1 to 20nm may be considered, or 0.1 to 10 nm, or 1 to 5 nm. Not only the sizeof structures, but also their material properties can influence theselection of wavelengths to use in investigations. For example, toperform reflectometry, at least background material of the structurerequires good reflection strength at the wavelength used. Forinvestigation of buried features, the wavelength should be chosen toobtain sufficient penetration through overlying material.

EUV metrology can be used to measure structures within the resistmaterial treated within the litho cell (After Develop Inspection orADI), and/or to measure structures after they have been formed in hardermaterial (After Etch Inspection or AEI). For example, substrates may beinspected using EUV metrology apparatus 244 after they have beenprocessed by the developing apparatus 212, etching apparatus 222,annealing apparatus 224 and/or other apparatus 226. X-ray techniqueswill generally be limited to AEI, due to poor contrast in the resistmaterial at x-ray wavelengths.

For high-volume manufacturing applications, a high brightness radiationsource would be desirable, to reduce acquisition time for eachmeasurement. The limited power of current compact x-ray sources meansthat known T-SAXS techniques suffer from a very low throughput,especially for small size metrology targets. This is especially the casewhen one seeks to obtain a very small spot size for illuminating a smalltarget area on a substrate. Known EUV sources are also limited inbrightness, and limited in selection of wavelengths. To maximizecontrast in target structures and to discriminate between structures ofdifferent materials, fine control of the wavelength over a wide rangewould be desirable.

The manufacturing system illustrated in FIG. 1 includes one or more EUVmetrology apparatuses 244 in addition to the optical scatterometer 240.This EUV metrology apparatus provides additional metrology results 246which can be used by supervisory control system SCS to achieve furthercontrol of quality and improvement in performance of the lithographicmanufacturing system as a whole. Like the optical scatterometer 240,metrology apparatus 244 can be applied at different stages inmanufacture, such as ADI and AEI, mentioned above.

EUV Reflectometry Method

FIG. 2 illustrates an metrology method while FIG. 3 illustrates anmetrology apparatus 300. The apparatus can be used as an example of EUVmetrology apparatus 244 for measuring parameters of substrates Wprocessed in the manufacturing system of FIG. 1. The apparatus can beused in wavebands other than EUV,

In FIG. 2, the target T is represented schematically as comprising aone-dimensional grating structure at the origin of a spherical referenceframe. Axes X, Y and Z are defined relative to the target. (Of courseany arbitrary coordinate system can be defined in principle, and eachcomponent may have its own local reference frame, that can be definedrelative to the one shown.) The direction of periodicity D of the targetstructure is aligned with the X axis. The drawing is not a trueperspective drawing, but a schematic illustration only. The X-Y plane isthe plane of the target and substrate, and for clarity is shown tiltedtoward the viewer, represented by an oblique view of circle 302. The Zdirection defines the direction N normal to the substrate. In FIG. 2, aray of incident radiation is labeled 304 and has an angle α of grazingincidence. In this example, the incident ray 304 (and all incident raysforming the radiation spot S) lie substantially in a plane parallel tothe X-Z plane, that is a plane defined the directions D and N andrepresented by circle 306. A reflected ray 308 that is not scattered bythe periodic structure of the target (i.e. a ray of specular reflection)emerges towards the right hand side of the target in the diagram, withan elevation angle α.

Other rays 310 are scattered at angles different to the specularreflection, in accordance with diffraction properties of the target. Theangle of separation between these rays and the specular ray will dependon the relationship between the wavelength of the radiation and thespacing of features of the target. The drawing is not to scale. Forexample, detector 312 may be closer to or further from the target thanshown, the target grating will likely be very small relative to thedetector; the angles of diffraction of rays 310 could be much wider thanindicated.

To perform reflectometry, the ray 308 and/or the scattered rays 310 arecaptured by a photodetector 312. Detector 312 comprises for example aposition-sensitive EUV detector, typically an array of detectorelements. The array may be a linear array, but in practice a2-dimensional array of elements (pixels) may be provided. Detector 313may be for example a CCD (charge coupled device) image sensor or a CMOSimage sensor. This detector is used to transform the reflected radiationinto electrical signals and eventually digital data for analysis. Asingle pixel detector may be sufficient in principle for some types ofmeasurement. More flexibility of operation will be allowed by having atwo-dimensional image detector.

From the measured spectrum, obtained for one or more wavelengths and oneor more values of incidence angle a, a measurement of a property of thetarget structure T can be calculated in a manner described furtherbelow.

EUV Reflectometry Apparatus

Turning to FIG. 3, metrology apparatus 300 is provided for measuringproperties of a metrology target T formed on substrate W, by the methodof FIG. 2. Various hardware components are represented schematically.The practical implementation of these components can be performed by therelevant skilled persons applying a mixture of existing components andspecially-designed components, according to well-known designprinciples. A support (not shown in detail) is provided for holding thesubstrate at a desired position and orientation relative to othercomponents to be described. A radiation source 330 provides radiation toan illumination system 332. Illumination system 332 provides a beam ofradiation represented by ray 304 which (together with other rays formingan illuminating beam) forms a focused irradiation spot on target T.Detector 312 and any ancillary optical components may be convenientlyconsidered as a detection system 333.

Substrate W in this example is mounted on a movable support having apositioning system 334 such that an angle of incidence a of ray 304 canbe adjusted. In this example, it is chosen as a matter of convenience totilt the substrate W to change the incidence angle, while the source 330and illumination system 332 remain stationary. In order to catch thereflected ray 308, detection system 333 is provided with a furthermovable support 336, so that it moves through an angle 2 a relative tothe stationary illumination system, or through an angle a relative tothe substrate. In the grazing incidence regime of reflectometry, it isconvenient to define the incidence angle α by reference to the plane ofthe substrate, as shown. Of course, it could equally be defined as anangle between the direction of incidence of incident ray 304 and adirection N normal to the substrate.

In alternative embodiments, the angle of incidence can be varied in morethan one dimension, for example by use of a conical mount. This type ofarrangement and its potential benefits are described in detail in theEuropean patent application number 15160786, mentioned above. The entirecontents of that application are incorporated herein by reference.

Additional actuators, not shown, are provided for bringing each target Tinto a position where the focused spot S of radiation is located.(Looking at it another way, to bring the spot to the position where thetarget is located.) In a practical application, there may be asuccession of individual targets or target locations to he measured on asingle substrate, and a succession of substrates too. It is immaterial,in principle, whether the substrate and target are moved and reorientedwhile the illumination system and detector stay still, or whether thesubstrate stays still while the illumination system and detector aremoved, or whether different components of the relative movement areachieved by a combination of these techniques. The present disclosureencompasses all these variants.

As already described with reference to FIG. 2, the radiation reflectedby target T and substrate W is split into a spectrum 310 of rays ofdifferent wavelengths, before it impinges on detector 312. A seconddetector 338 will normally also be provided for measuring intensity ofthe incident beam, for use as a reference. A processor 340 receivessignals from the detectors 312 and 338. The resulting reflection datafor one or more angles of incidence are used in the processor tocalculate a measurement of property of the target, for example CD oroverlay.

Inverse Compton Scattering Radiation Source

FIG. 4 is a schematic block diagram of the radiation source 330 in theapparatus of FIG. 3. To provide metrology apparatus 300 that can be usedin the semiconductor industry as a primary metrology tool with highthroughput and/or high density of measurements per wafer, a very brightsource is desirable. For inspection of product structures in EUV, awavelength range around 13 nm is relevant, similar to the wavelengthused for EUV imaging in the latest lithographic apparatus. However, inthis wavelength range, compact high brightness sources are not availableon the market today. The inventors have identified that a source basedon Inverse Compton Scattering (ICS) can be developed into a sourcesuitable for use as source 330 in the apparatus 300. An ICS-based sourceis expected to be able to provide the high brightness needed in EUVrange for scatterometry and reflectometry of different kinds. The sameor a similar source can also provide radiation in the X-ray range forGI-SAXS or T-SAXS applications. A tunable source in the range of, forexample, 0.01 nm to 125 nm with high brightness can be envisaged. Thiswould be beneficial for scatterometry (fixed angle, or combined withangular scan). If the source produces radiation with a certain level ofspatial coherence, it can also be used for so-called lensless imagingtechniques (coherent diffraction imaging (CDI), including for exampletechniques known as ankylography and ptychography).

Radiation source 330 in this example is based around interaction betweena high energy electron beam 400 and laser radiation 401. A briefdescription of the main components and operation will be provided here.For more detail of the implementation of an ICS source, reference ismade to W S Graves et al, “Compact x-ray source based on burst-modeinverse Compton scattering at 100 kHz”, Physical Review SpecialTopics—Accelerators and Beams 17, 120701 (2014), The contents of theGraves et al reference are incorporated herein by reference.

Components associated with the electron beam are a radio frequency (RF)electron gun 402, a linear accelerator (linac) 404, a focusing assembly406, a beam deflector 408 and an electron dump 410. Electron gun 402contains a photocathode and accelerator, so that bunches of electronscan be emitted into linac 404 when triggered by optical pulses 412 froma photocathode laser 414. Focusing assembly 406 may for example comprisea set of quadrupole electromagnets. Deflector 408 may comprise a dipoleelectromagnet.

A seed laser 416 provides pulses of seed radiation to photocathode laser414. In the example described by Graves et al, these are pulses of 1030nm wavelength, with a pulse frequency of 200 Mhz. Photocathode laser 414in operation selects groups of pulses, amplifies them optically, andconverts them by fourth harmonic generation to pulses of approximately250 nm wavelength. These pulses are the ones delivered to thephotocathode to generate electron bunches. Pulses may be selected forexample with a frequency of 100 kHz, 100 pulses per group, and 1000groups per second. Linac 404 accelerates the electrons to an energy inthe range, for example, 8 MeV to 40 MeV. Electron beam 400 is thusgenerated and delivered with a well-defined energy per electron, tointeraction point IP. The beam is focused to its narrowest point at theinteraction point labeled IP. After passing the interaction point,deflector 408 steers the electron beam to an electron dump 410.

Laser beam 401 is generated by ICS laser 420. This, too, is seeded bypulses picked from the output of seed laser 416. Pulses are taken at 1kHz, amplified and compressed to generate pulses of 3 picosecond pulselength at 1 kHz, with wavelength 1030 nm. An output beam 422 of ICSlaser 420 is delivered into an optical cavity 424 formed between twomirrors 426, 428. An enlarged detail of the optical cavity and theinteraction point is shown in FIG. 5.

Referring to FIGS. 4 and 5 together, mirror 426 at the input end of thecavity is a dichroic mirror, so as to pass certain wavelengths andreflect others. Mirror 428 is a totally reflecting mirror. Cavity 424also contains a lens 430 and a harmonic generator 432. Harmonicgenerator 432, for example a crystal of lithium triborate (LBO) orbarium borate (BBO), converts a proportion of the incoming photons inbeam 422 to a wavelength of 515 nm, by frequency doubling (secondharmonic generation). By the action of dichroic mirror 426, thesephotons are trapped in the cavity, to form the beam 401 that interactswith the electron beam 400. Mirror 426 is also curved to provide afocusing function. Lens 430 and mirror 426 focus the trapped laser beam401 so as to define a beam waist at the desired interaction point IP. Inthe practical example, the laser beam 410 is aligned as closely aspossible to the direction of the electron beam 400, while still avoidinginterference between the electron beam and the laser optics. Graves etal use an angle of 50 mrad (millirad), for example. The angle isexaggerated in the drawings, which are not to scale.

By these means, it is arranged that a train of 100 electron bunchesarrive at the interaction point IP 100 times per second, at the sametime as a pulse of intense laser radiation is present. inelasticscattering between the electrons and the photons in the two beamstransfers additional energy to the photons, so that a proportion of themachieve the desired frequency to generate a desired output beam 440 ofphotons having the desired wavelength for use in the metrology method ofFIG. 2. Output beam 440 has a relatively small divergence of, forexample 10 mrad, and a reflective focusing optic 442 is provided tofocus the output beam to the desired radiation spot S. In that regard,focusing optic 442 may be regarded forms part of the illumination system332 of FIG. 3, and/or as part of the source 330. For use at EUVwavelengths, in the present example, optic 42 may comprise a multilayermirror, or a simple metal mirror of for example Au or Ru metal. Focusingoptic 442 can also serve as a beam splitter, to provide referenceillumination to the detector 338 (not shown in FIG. 4). Beam splittingcan be performed for example by a grating structure integrated in thefocusing optic. Beam splitting can be performed by a separate element,if preferred.

Returning to the generation of electron beam 400, electric power for thegeneration of the electron beam 400 is provided at radio frequency by aklystron 450 and waveguide system 452. The voltage and current of theseare controlled by a control system 454. The voltage on the linacdetermines the acceleration and final energy of the electrons in beam400. This directly influences the energy imparted to the laser photons,and hence determines the wavelength of the radiation forming output beam440. In the example of Graves et al, an x-ray source is desired andphotons with energy 12.4 keV are produced, corresponding to a wavelengtharound 0.1 nm in the x-ray band. Such an x-ray source can be useful insemiconductor metrology, but only in grazing incidence. The source maybe usable for example for T-SAXS and may be more suited to high volumemeasurements than conventional plasma sources (which are not as bright)or synchrotron sources (which are too large and expensive). However,x-rays in grazing incidence are not really suitable for investigatingsurface structures within a small localized area on a wafer. With anenergy of 13 keV or more, the x-rays can be used in transmission througha silicon wafer. The present inventors have further recognized that theICS source, while designed for generating hard x-rays, can be adaptedand adjusted to produce lower energy photons (longer wavelengths),simply by lowering the energy of the electrons. In the present example,this is achieved by reducing the voltages supplied to the linac, whichcan be done very quickly by suitable design of the control system 454and other components. Equivalent steps to adjust the electron energy canbe applied instead of or in addition to varying the linac voltage, or inthe case of sources not using a linac.

The source 330 can therefore be used to produced soft x-ray or EUVradiation with high brightness, instead of or as an alternative to“hard” x-rays. If the source can also be adjusted to produce hardx-rays, that will be a bonus. For the primary purpose envisaged herein,EUV radiation can be produced for example in an energy (wavelength)range that includes 1 keV (around 1.2 nm), 0.1 keV (12.4 nm) and even0.01 keV (124 nm). These wavelengths, which also correspond with thedimensions of the product features whose parameters are to be measured,permit large incidence angles and therefore finer focused spots.

Additionally, while conventional plasma sources are very variable inbrightness over the EUV spectrum, the ICS source described here can betuned to virtually any desired frequency in the band of interest, bysuitable control of the voltage and or other parameters around theelectron gun and the linac. Not only can the wavelength be changed frommeasurement to measurement, but it can also be changed within ameasurement, for example to obtain “spectroscopic” information.

Furthermore, by reducing the electron energy further and further, thesame source may in principle operate at even longer wavelengths, intoso-called VUV (100-200 nm or 125-200 nm, for example) and UV (200-350nm), and even visible (400-800 nm) wave bands. The reflective optic 442,while effective at EUV wavelengths, can serve equally to focus radiationup to the visible wavelength range. The ICS source would be excessivelycomplicated and expensive, if one only wanted a source of UV or visibleradiation. However, to have a single source and optical system operableto produce such a wide variety of wavelengths, substantially byelectronic control changes only, allows new flexibility in the metrologyapparatus 300. Such an apparatus can be useful not only in semiconductormanufacturing, but in a wide range of applications of scientificinvestigation and/or quality control applications.

Optical system for EUV Reflectometry

Example applications of this source 330 include EUV reflectometry, usingan apparatus such as the one illustrated in FIG. 3. In the applicationof this EUV-SR to metrology in semiconductor manufacturing, smallgrating targets can be used. Multiple diffraction spectra are capturedusing detector 312, while setting the grazing angle of incidence a tovarious different values. Using the detected spectra and a mathematicalmodel of the target structure, reconstruction calculations can beperformed to arrive at measurement of CD and/or other parameters ofinterest. An example reconstruction method will be illustrated furtherbelow.

Considering the target illustrated in FIG. 2 as an example, dimensionsof the lines and spaces will depend on the target design, but the periodof the structure may be for example less than 100 nm, less than 50 nm,less than 20 nm, even less than 10 nm and down to 5 nm. The lines of thegrating structure may be of the same dimension and pitch as productfeatures in a product area of the substrate. The lines of the gratingstructure may in fact be the lines of a product structure, rather than atarget structure formed, within a dedicated target area, solely for thepurposes of metrology. Such small features may be formed for example inan EUV lithography process, by imprint lithography or by direct-writemethods. Such small features may also be formed using present-day INNlithography, by a so-called double-patterning processes (generallymultiple-patterning). Techniques in this category includepitch-doubling, for example by litho-etch-litho-etch (LELE) andself-aligned dual-damascene in back end-of the line (BEOL) layers. Forthe purposes of explanation, it will be assumed in the followingexamples that CD is the parameter of interest. However, where there aretwo gratings formed on top of one another, another parameter of interestmaybe overlay. This can be measured based on asymmetry in the EUV-SRdiffraction orders, as described separately below. The incidence anglecan be elevated if necessary to achieve adequate penetration to thelower structure.

In the multiple-patterning process, structures are formed in one layerof the product not in one patterning operation but in two or morepatterning steps. Thus, for example, a first population of structuresmay be interleaved with a second population of structures, and thepopulations are formed in different steps, so as to achieve a higherresolution than one step alone can produce. While the placement of thepopulations should be identical and perfect in. relation to otherfeatures on the substrate, of course every real pattern exhibits acertain positional offset. Any unintentional positional offset betweenthe populations can be regarded as a form of overlay, and can bemeasured by analogous techniques to those used to measure overlaybetween layers. Additionally, overlay against features in an underlyingor overlying layer can be different for each population when multiplepopulations of features are formed in a single layer, and overlay foreach of these populations can be measured separately if desired.

FIG. 6 illustrates the variation of elongation of a radiation spot,which can be challenging for implementation of in-die metrology usinggrazing incidence reflectometry, In FIG. 6(a), the substrate W andtarget T are shown in cross-section. Representative incident ray 304 andreflected ray 308 are illustrated, with incidence angle α relative tosubstrate W. As these are representative rays, it should be consideredthat the incident radiation as a whole comprises many rays, which form abeam indicated schematically at 602. Similarly, the reflected radiationcomprises many rays 308 which form a beam indicated schematically at604. In order to make use of the smallest possible target, a radiationspot is formed by focusing the rays of the beam 602, so that theyconverge to define a minimum beam diameter precisely where they meet thesurface of substrate W. In the illustration, the incident beam 602 isconvergent to a focus with a minimum diameter d_(B). The reflected beam408 (ignoring scattering effects) comprises divergent rays, as shown.Because grazing incidence angle α is relatively small (in other words,nearer to zero than to 90°) the diameter d_(S) of the radiation beam602, as projected on to the target T, is several times greater than thebeam diameter d_(B). The ratio between diameters d_(S) and d_(B) dependson the sine of angle α as shown in FIG. 4(a).

As shown in FIG. 6(b), to achieve circular spot S that fits within thearea of target T, the beam 602 should have the strongly elliptical crosssection shown at B. When the angle α is 5°, for example, the minimumdiameter d_(B) of the beam should be more than ten times smaller thanthe allowable diameter of the spot d_(S) (sin 5°=0.087). For lowerangles of incidence, the minimum diameter of the beam would have to betens, hundreds or even thousands of times smaller. To obtain a spot thatfits within a small target area such as 5 μm square would be impossiblein practice. Even at α=5°, the minimum beam diameter d_(B) should bearound 436 nm to achieve a spot size under 5 μm. Conversely, as seen inFIG. 6(c), an increase in the grazing incidence angle α greatly relaxesthe minimum diameter requirement of the beam 404. The ellipse B′ can bemuch broader than the ellipse B, in order to achieve a spot S′ that fitswithin the area of target T. For example, for α=20°, the beam diameterwill be increased only by a factor of three. The minimum diameter d_(B)can be as large as 1.7 μm without exceeding the 5 μm spot size.

Compared with known techniques, particularly X-ray reflectometry(GI-XRS), the use of EUV wavelengths in the range 1 to 100 nm allowsthese higher incidence angles and can bring smaller spot sizes withinthe capability of EUV optical design. This capability of the source 330described above enables EUV reflectometry to be considered for measuringsmall targets on substrates in high volume manufacturing.

FIG. 7 illustrates one possible arrangement of the illumination system332 in the apparatus of FIG. 3, and part of detection system 333 also.Radiation source 330 based on inverse Compton scattering producesradiation in a well-defined beam 440 of radiation, as described abovewith reference to FIGS. 4 and 5. Some or all of the elements of theoptical system shown may be omitted if unnecessary to achieve a desiredperformance with this source.

Beam 440 of EUV radiation having a desired wavelength is received with acertain divergence. At the exit of the source 330 (the entrance to theillumination system 332), a first aperture 620 is provided to serve asan entrance pupil for the illumination system. An incoming beam 622 withlesser divergence impinges on a focusing optical element or system. Thisfocusing system is implemented in the present illustration by a2-dimensionally curved mirror 624, for example an ellipsoidal mirror.Mirror 624 produces a convergent beam 626, which is focused to form thespot at the target location on substrate W. Optionally, a secondaperture 630 is provided to restrict the diameter of beam 632 at thetarget. In particular, aperture 630 may be made adjustable in heightand/or width so that different shapes of beam B′ can be generatedaccording to different desires/sizes, and different incidence angles α.It will be understood that curved mirror 624 corresponds to the focusingoptic 442 shown in FIG. 4.

Reflected beam 634 enters detection system 332, carrying informationabout the structure of the target to detector 312 (not shown in thisview). Optionally, a second focusing mirror 636 is provided to reducedivergence of the beam, as it enters detection system 333. Either of thetwo-dimensionally curved mirrors 624, 636 could be replaced by a seriesof two or more one-dimensionally curved (cylindrical) mirrors. Asillustrated, mirror 624 can also serve as a beam splitter, to providereference illumination to the detector 338. Beam splitting can beperformed for example by a grating structure integrated in the mirror.Other variations in the optical system are of course possible, withoutdeparting from the principles of the present disclosure.

Wavelength Selection

The ability to make measurements at different wavelengths can greatlyincrease information diversity which aids processor PU to measureaccurately nanostructure using methods such as optical scatterometry. Itis possible with the ICS source to select the wavelength and set it upclose to or at absorption edge of a specific material enabling thepossibility to do material dependent scatteromeuy or imaging, forinstance.

FIG. 8 illustrates the interaction of radiation of different wavelengthswith various materials, which are just a selection of those encounteredin a typical semiconductor product. In four graphs the horizontal axisrepresents wavelength λ on a logarithmic scale, running from 0.1 nm to40 nm, just for the sake of example. As mentioned already, thesewavelengths encompass the dimensions of the features of interest, andthe resolution of measurement desired. The vertical axis in each graphrepresents penetration depth (attenuation length) in pm, or a materialof given density. The absolute scale is not relevant, but what can beseen is that each material exhibits strong differences in attenuation atwavelengths either side of certain “edges”, and that these edges fall atdifferent wavelengths for different elements and compounds.

For example, in the top graph, silicon Si exhibits a strong edge 700 ataround 12.4 nm. Measurements at the wavelength 702 are relativelysensitive to the presence of silicon, while measurements at thewavelength 704 are relatively insensitive. In the second graph, oxygenexhibits a different edge 710 and in the third graph the oxide SiO₂exhibits edges 720, 722 corresponding to both Si and O components.Carbon in the fourth graph has an edge 730 at yet another wavelength.Accordingly, it will be seen that a resist structure (high in carbon)can be measured more accurately by selecting wavelength 732, rather thansome arbitrary wavelength. For further accuracy, at the expense of anadditional illumination and detection step, differential measurementscan be obtained by detecting a scatter patterns or reflectances atwavelengths 732 and 734, either side of the edge 730. Similardifferential pairs of wavelengths are illustrated either side of theedges 700 and 710. The same measurement strategy is applied to Ge, forexample.

An application of an ICS source, which takes advantage of the tunabilityof the emitted illumination's wavelength, is a metrology applicationwhere wavelengths from visible to EUV ranges are used to provideinformation about parameters of a lithographic process, whereaswavelengths produced by ICS source, shorter than typical EUVwavelengths, are used for measurements related to characterizing thematerial properties. For example, roughness or topology of theunderlying structure is measured while changing the wavelength of theICS source. For longer wavelengths, roughness information is scarcelypresent in the reflected illumination, while shorter wavelengths aremore sensitive to the local variations in topology, such as surfaceroughness. It is also found that there is a threshold in theillumination wavelengths where the topology information is no longerpresent. Further material related information which is obtained forshorter wavelengths is the material stoichiometry, density, electricaland/or optical properties such as conductivity and capacitance.

Compared with available plasma-based EUV sources, the controllablevariation in wavelength using the ICS technique will not lead to widevariations in source brightness. The trade-offs and compromisesassociated with known sources can be largely avoided.

Application Examples in High Volume Manufacturing

FIG. 9 illustrates a substrate W which is one of a batch or lot ofwafers undergoing processing in the facility of FIG. 1. One or morescatter patterns are captured with the detector at each target location900. The scatter patterns are used by processor 340 to calculate ameasurement and reported to the operator, LACU 206 and/or SCS 238. Whilesemiconductor substrates (wafers) are cited as a particular type ofproduct to be measured using an inspection apparatus with an ICS sourceof the type described herein, it should be understood that thecapabilities of the apparatus with this source are applicable in a widerange of inspection and measurement tasks.

While a target structure I in the form of a grating has been illustratedand described above, the methods enabled by the present disclosure canbe adapted to are not limited to use with periodic target structures,nor to structures that are dedicated to metrology. The target structurecould be part of a product structure. Product structures for thispurpose includes not only structures pre-sent in a finished product, butalso structures present at intermediate stages in a manufacturingprocess, such as a resist pattern, or a hard mask.

While a product in the form of a processed semiconductor substrate hasbeen illustrated, another product requiring inspection is the mask orreticle that is used as a patterning device in the lithographicapparatus. The target structure may be part of such a patterning device.Inspection may be performed for quality control during and aftermanufacture of the patterning device. Inspection may be performedperiodically during use of the patterning device, for example to detectdamage or contamination.

The structures under inspection in the examples have patterns appliedand structures formed in accordance with the patterns. However, themethods of the present disclosure can be applied also to inspection ofblank substrates for semiconductor products or patterning devices.Inspection in that case can be for measurement of layer thicknesses orcomposition, and/or uniformity and/or for detection of defects such asdamage and contamination.

FIG. 10 illustrates the same apparatus performing an enhancedmeasurement method, in which multiple scatter patterns are captured withthe detector at each target location, or at different target locationsswitching the wavelength each time. The results of these differentwavelengths are combined by processor 340 into a single measurement andreported to the operator, LACU 206 and/or SCS 238. The number ofdifferent wavelengths can be as few as two, or it could be ten or more.The high brightness of the ICS source, together with the ability toswitch wavelength in under a second, under half a second or even under atenth of a second, allows these multiple measurements to be made withhigh throughput.

Using the example of spectroscopic reflectometry method, the purpose ofthe metrology techniques is to calculate measurements of one or moreparameters of the shape. Applying, for example, a reconstructiontechnique, rigorous optical theories are used effectively to calculatewhat values of these parameters will result in a particular observedreflection spectrum (optionally including spectra of one or more higherdiffraction orders). In other words, target shape information isobtained for parameters such as CD (critical dimension) and overlay. CD,or critical dimension, is the width of the object that is “written” onthe substrate and is the limit at which a lithographic apparatus isphysically able to write on a substrate. In some situations, theparameter of interest may be CD uniformity, rather than an absolutemeasurement of CD itself. Other parameters such as grating height andside wall angle may also be measured, if desired. Overlay metrology is ameasuring system in which the overlay of two targets is measured inorder to determine whether two layers on a substrate are aligned or not.

Using results from EUV metrology apparatus 300 in combination withmodeling of a target structure such as the target T and its reflectionand/or diffraction properties, measurement of the shape and otherparameters of the structure can be performed in a number of ways. In afirst type of process, a diffraction pattern based on a first estimateof the target shape (a first candidate structure) is calculated andcompared with the observed. reflection pattern at different wavelengths.Parameters of the model are then varied systematically and thereflection spectra re-calculated in a series of iterations, to generatenew candidate structures and so arrive at a best fit. In a second typeof process, reflection spectra for many different candidate structuresare calculated in advance to create a ‘library’ of reflection spectra.Then the reflection spectrum observed from the measurement target iscompared with the library of calculated spectra to find a best fit. Bothmethods can be used together: a coarse fit can be obtained from alibrary, followed by an iterative process to find a best fit. It isexpected that in EUV spectroscopic reflectometry the computation for thefirst type of process will not be burdensome. That being the case, therewould be no need to resort to a library process.

These types of process are known in principle to the skilled reader, andcan be adapted to exploit the metrology apparatus 300 with ICS radiationsource.

FIG. 11 illustrates the same apparatus performing a different enhancedmeasurement method, in which multiple scatter patterns are captured withthe detector at each target location, or at different target locationsswitching the wavelength each time. The results of these differentwavelengths are used by processor 340 to obtain different measurementsof different parameters. For example, a conventional metrology targetcould be measured with a visible wavelength radiation, and then aproduct structure measured with EUV radiation. Again, the results arereported to the operator, LACU 206 and/or SCS 238.

Potentially the dedicated optical scatterometer 240 of FIG. 1 could bereplaced by functions included within the EUV metrology apparatus 244.

Applications and benefits of such hybrid metrology techniques aredisclosed in our European patent application 14168067.8, filed on 13 May2014, not published at the present priority date. In that example, anoptical scatterometer is used to measure one type of target, while anx-ray metrology apparatus is used to measure other targets by T-SAXS. Inthe present disclosure, the same source can be used for bothmeasurements.

FIG. 12 illustrates a combination of the principle of FIGS. 10 and 11,in which multiple types of measurement are made on the same or differenttargets, each type of measurement using multiple wavelengths on the sametarget.

All of these different modes of operation are greatly facilitated by theprovision of a high brightness source, switchable rapidly betweendifferent wavelengths across the EUV waveband and optionally overshorter and/or longer wavebands.

FIG. 13 illustrates application of the source 330 in a T-SAXS metrologyapparatus 1300. This is similar to the apparatus of FIG. 3, except thatthe substrate is presented at or near normal incidence to the radiationbeam 1304, and the detected 1312 is located behind the substrate. Otherreference signs beginning ‘13’ are similar in effect to those beginningwith ‘3’ in FIG. 3. If the source generates x-rays, it can be used forT-SAXS. For silicon products, photon energies greater than 13 keV willbe most effective, and currently the energy used in T-SAXS is 17keV).However, even the energy 12.4 keV obtained in the example of Graves etat could be high enough (given the source brightness) to make T-SAXSpracticable. Depending on the mechanical arrangements, potentially thepositioning system 1334 could move a substrate between the reflectometryposition shown in FIG. 3 and the transmission position shown in FIG. 13,within the same instrument. The detectors 340 and 1340 could be separatedetectors, to simplify the mechanical design. Again a reference detector(not shown) can be provided to measure intensity of the illumination inreal time.

FIG. 14 illustrates the application of a measurement method, for examplethe methods described above, in the management of a lithographicmanufacturing system. The steps will be listed here, and then explainedin more detail:

S21: Process wafer to produce structures on substrateS22: Measure CD and/or other parameter across substrateS23: Update metrology recipeS24: Update lithography and/or process recipe

At step S21, structures are produced across a substrate using thelithographic manufacturing system. At S22, the EUV metrology apparatus244 (for example metrology apparatus 300) and optionally other metrologyapparatus and information sources are used to measure a property of thestructures across the substrate. At step S23, optionally, metrologyrecipes and calibrations of the metrology apparatus and/or othermetrology apparatus 240 are updated in light of the measurement resultsobtained.

At step S24, measurements of CD or other parameters are compared withdesired values, and used to update settings of the lithographicapparatus and/or other apparatus within the lithographic manufacturingsystem. By providing an EUV metrology apparatus with high volumethroughput, the performance of the whole system can be improved. Productfeatures and/or product-like features can be measured directly, even atthe smallest technology nodes, and in-die targets can be provided andmeasured without losing too much area.

In the above steps, it is assumed that sufficient targets are measuredacross a substrate and across multiple substrates, that statisticallyreliable models of the process are derivable. The profile of CD andother parameters does not need to be expressed entirely as a variationacross the substrate. It can be expressed for example as an intra-fieldprofile that is common to all fields (each instance of patterning usingthe patterning device M at a different location on the substrate W) anda lower order, inter-field, variation onto which the intra-fieldvariation is repeatedly superimposed. The settings of the lithographicprocess adjusted in step S24 can include intra-field settings as well asinter-field settings. They may be applicable to all operations of theapparatus, or specific to a particular product layer.

CONCLUSION

Based on the described examples metrology apparatus using a ICS-basedsource, the skilled reader will appreciate that the same type of sourcecan be used in a variety of applications and metrology systems, not onlyEUV spectroscopic reflectometry and EUV reflectometry. For example:

-   -   If the source generates x-rays, it can be used for T-SAXS, as        already illustrated.    -   GI-SAXS becomes more practicable: considering that loss of        photons can be huge when trying to limit the spot size at such        shallow incidence angles. The brightness and angular spread of        the ICS source means that GI_SAXS may also become practicable in        a high-volume environment.    -   If the ICS source can be controlled to produce radiation with a        sufficient spatial coherence, additional metrology technique        become available. Coherent diffraction imaging (CM) method        become of interest, such as ptychography. The high source        brightness becomes useful, as well as the selection of        wavelengths, because in ptychography (for example) multiple        images have to be acquired in that technique to retrieve the        phase of the scattered wave. Similarly other CDI techniques        require high resolution data capture, for which high brightness        is also a key enabler.    -   As mentioned, radiation in VUV, DUV, UV and visible ranges can        be generated by providing suitable control and power supply        arrangements for the electron gun and linac.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. In association with the novel targets as realized onsubstrates and patterning devices, an embodiment may include a computerprogram containing one or more sequences of machine-readableinstructions describing methods of producing targets on a substrate,measuring targets on a substrate and/or processing measurements toobtain information about a lithographic process. This computer programmay be executed for example within unit PU in the apparatus of FIG. 3and/or the control unit LACU of FIG. 2. There may also be provided adata storage medium (e.g., semiconductor memory, magnetic or opticaldisk) having such computer program stored therein.

Although patterning devices in the form of a physical reticle have beendescribed, the term “patterning device” in this application alsoincludes a data product conveying a pattern in digital form, for exampleto be used in conjunction with a programmable patterning device.

Further embodiments according to the present invention are provided inbelow numbered clauses:

-   -   1.A method of measuring a property of a structure, the method        comprising irradiating the structure at least a first time with        radiation, detecting said radiation after interaction with the        structure and determining a property of the structure based on a        property of the radiation, wherein said radiation is generated        by inverse Compton scattering, the radiation having a first        wavelength in the range of 0.1 to 125 nm.    -   2. A method according to clause 1 wherein the structure is        irradiated with said radiation using a beam diameter an extent        less than 10 μm, optionally less than 5    -   3. A method according to clause 1 or 2 wherein the same        structure is irradiated and radiation detected at least a second        time, the radiation the second time being generated by inverse        Compton scattering and having a second wavelength within the        range 0.1 to 125 nm.    -   4. A method according to clause 3 wherein the property of the        structure is determined based on the detected radiation of both        the first and second wavelengths.    -   5. A method according to clause 1, 2, 3 or 4 wherein the same or        a different structure is irradiated and radiation detected at        least a second time, the radiation the second time being        generated by inverse Compton scattering and having a second        wavelength outside the range 0.1 to 125 nm.    -   6. A method according to clause 5 wherein the second wavelength        is shorter than 0.1 nm.

7. A method according to clause 5 wherein the second wavelength islonger than 125 nm.

-   -   8. A method according to clause 5 wherein the second wavelength        is longer than 200 nm.    -   9. A method according to clause 5 wherein the second wavelength        is longer than 350 nm,    -   10. A method according to any of clauses 3 to 9 wherein said        first and second times are completed in less than one second.    -   11. A method according to any preceding clause wherein said        radiation is generated by delivering a beam of electrons and a        beam of photons simultaneously to an interaction point.    -   12. A method according to clause 11 further comprising adjusting        the wavelength of the radiation one or more times by adjusting        an energy of electrons in the electron beam.    -   13. A method according to clause 11 or 12 wherein said beam of        electrons is generated using an electron gun and a linear        accelerator.    -   14. A method according to any preceding clause wherein said        structure is formed on a semiconductor substrate.    -   15. A method according to clause 14 wherein a direction of said        irradiation is greater than 2° from a direction parallel to the        substrate.    -   16. A radiation source apparatus based on inverse Compton        scattering, the apparatus comprising an electron source and a        photon source, and a controller for controlling the electron        source and the photon source to deliver one or more bunches of        electrons to an interaction point at the same time as a pulse of        photons, whereby a proportion of the photons acquire additional        energy by inverse Compton scattering to and are output by the        apparatus, wherein the additional energy is controllable such        that the photons output by the apparatus have a wavelength in        the range 0.1 nm to 125 nm.    -   17. An apparatus according to clause 16 wherein the additional        energy is controllable such that the photons output by the        apparatus have a wavelength freely selectable over at least a        sub-range of the range 0.1 nm to 100 nm.    -   18. An apparatus according to clause 16 or 17 wherein the        additional energy is further controllable such that the photons        output by the apparatus at another time have a wavelength        outside the range 0.1 nm to 100 nm.    -   19. A metrology apparatus for use in measuring properties of a        structure, the apparatus comprising:

a radiation source apparatus according to any of clauses 16 to 18;

an illumination system for delivering photons output by the radiationsource apparatus in a beam of radiation onto the structure; and

a detection system for detecting radiation from the structure after saidphotons have interacted with the structure.

-   -   20. An apparatus according to clause 19 wherein the beam of        radiation when projected onto the structure has an extent less        than 10 pm, optionally less than 5 p.m.    -   21. An apparatus according to clause 19 or 20 further comprising        a controller for changing the wavelength of the radiation in        said radiation beam while detecting said radiation multiple        times.    -   22. An apparatus according to clause 21 wherein the controller        is operable o set a new wavelength and detect radiation at least        two times in one second.    -   23. An apparatus according to any of clauses 19 to 22 wherein        said detection system is arranged to detect said radiation after        reflection from said structure.    -   24. An apparatus according to clauses 23 wherein the        illumination system and detection system are adapted for        measuring properties of structures formed at various locations        on a semiconductor substrate, and wherein an incidence angle of        said beam of radiation is greater than 2° relative to a surface        of the substrate    -   25. A device manufacturing method comprising:

transferring a pattern from a patterning device onto a substrate using alithographic process, the pattern defining at least one structure;

measuring one or more properties of the structure to determine a valuefor one or more parameters of the lithographic process; and

applying a correction in subsequent operations of the lithographicprocess in accordance with the measured property,

wherein the step of measuring the properties of the structure includesmeasuring a property by a method according to any of clauses 1 to 15.

-   -   26. A device manufacturing method according to clause 33 wherein        said functional device pattern defines product features having a        critical dimension less than 50 nm, optionally less than 20 nm.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography, atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used in relation to the lithographicapparatus encompass all types of electromagnetic radiation, includingthe ranges identified above.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1. A method of measuring a property of a periodic structure, the methodcomprising: generating a beam of radiation by inverse Comptonscattering; directing the beam at the periodic structure; detectingradiation transmitted and diffracted by the periodic structure; anddetermining the property of the periodic structure based on an asymmetryof diffraction orders of the detected radiation, and wherein the beamhas a wavelength between 0.1 to 125 nm.
 2. The method of claim 1,wherein the beam has a wavelength between 5 to 125 nm.
 3. The method ofclaim 1, wherein the beam has a wavelength between 10 to 100 nm.
 4. Themethod of claim 1, wherein the determining comprises determining anoverlay difference between a feature on a first layer of the periodicstructure and a feature on a second layer of the periodic structurecorresponding to the feature on the first layer generated bylithographic processes.
 5. The method of claim 1, wherein thedetermining comprises determining one or more of damage andcontamination resulting from a lithographic process.
 6. The method ofclaim 1, wherein the determining comprises determining one or more of astructure size, a line thickness, and critical dimension generated by alithographic process.
 7. The method of claim 1, further comprising:generating another beam of radiation by inverse Compton scattering;directing the another beam at the periodic structure; and detectinganother radiation transmitted and diffracted by periodic structure,wherein the another beam has a wavelength between 0.1 to 125 nm.
 8. Themethod of claim 7, wherein the directing the beam and the another beamand the detecting the radiation and the another radiation transmittedand diffracted by the periodic structure are completed in less than onesecond.
 9. The method of claim 1, further comprising: generating anotherradiation by inverse Compton scattering; directing the another beam atanother periodic structure; and detecting radiation transmitted anddiffracted by the another periodic structure, wherein the another beamhas a wavelength between 0.1 to 125 nm.
 10. The method of claim 1,wherein the generating comprises delivering a beam of electrons and abeam of photons simultaneously to an interaction point.
 11. The methodof claim 10, further comprising adjusting the wavelength of the beam ofradiation by adjusting an energy of electrons in the beam of electrons.12. The method of claim 10, wherein the beam of electrons is generatedusing an electron gun and a linear accelerator.
 13. A metrologyapparatus comprising: a radiation source comprising; an electron sourceconfigured to output electrons; a photon source configured to outputphotons, wherein the electrons and photons arrive at an interactionpoint simultaneously, wherein the photons have a wavelength between 0.1to 125 nm after undergoing inverse Compton scattering, and wherein theradiation source is configured to output the photons; an illuminationsystem configured to direct the photons output by the radiation sourceat a periodic structure; and a detection system configured to detectradiation transmitted and diffracted by the periodic structure, whereinthe metrology apparatus is configured to determine a property of theperiodic structure based on at least an asymmetry of diffraction ordersof the detected radiation.
 14. The metrology apparatus of claim 13,wherein the photons have a wavelength between 5 to 125 nm afterundergoing inverse Compton scattering.
 15. The metrology apparatus ofclaim 13, wherein the photons have a wavelength between 10 to 100 nmafter undergoing inverse Compton scattering.
 16. The metrology apparatusof claim 13, wherein the determined property of the periodic structurecomprises an overlay difference between a feature of the periodicstructure and a feature on the second layer of the periodic structurecorresponding to the feature on the first layer generated bylithographic processes.
 17. The metrology apparatus of claim 13, whereinthe determined property of the periodic structure comprises one or moreof a structure size, a line thickness, a critical dimension, a layercomposition, a layer thickness, damage, and contamination resulting froma lithographic process.
 18. The metrology apparatus of claim 13, furthercomprising a controller configured to adjust an energy of the electronsto adjust the wavelength.
 19. The metrology apparatus of claim 18,wherein the metrology apparatus is configured to cause, at least twotimes in one second, the controller to adjust the wavelength and thedetector to detect the radiation transmitted and diffracted by theperiodic structure.
 20. A lithographic apparatus comprising: anillumination apparatus configured to illuminate a pattern on apatterning device; a projection system configured to project an image ofthe pattern onto a substrate; a metrology apparatus comprising: aradiation source comprising; an electron source configured to outputelectrons; a photon source configured to output photons, wherein theelectrons and photons arrive at an interaction point simultaneously,wherein the photons have a wavelength between 0.1 to 125 nm afterundergoing inverse Compton scattering, and wherein the radiation sourceis configured to output the photons; an illumination system configuredto direct the photons output by the radiation source at a periodicstructure; and a detection system configured to detect radiationtransmitted and diffracted by the periodic structure, wherein themetrology apparatus is configured to determine a property of theperiodic structure based on at least an asymmetry of diffraction ordersof the detected radiation.